G31X Mini Geared Hubs

There are countless manufacturers and models of geared hub motors for ebikes, normally categorized as 250W or 350W machines. These small geared motor are often ideal for ebike conversions where the rider does not require large amounts of assist. They are lightweight, discrete due to their small diameter, and generally both inexpensive and well built.

We've stocked a variety of motors of this class in the past (Tongxin, Outrider, G01) but now carry exclusively the G310 (rear) and G311 (front) motors for this application. These little hubs really stand out from the rest.

Key Features

  1. Totally silent drive: We've grown accustomed to geared motors emitting a buzz while running. It's not that loud, but it does detract from the blissful peace you can expect while riding a bicycle in quiet neighborhoods. Well the G310/311 motors are completely smooth and silent over their entire speed range due to helical gears in the first reduction stage. If you've been put off geared motors because of the noise, these little hubs will change your perspective. They are smooth as silk.
  2. Side Cable Exit: These motors have an axle cut for side exit of the motor cable rather than running through the end. This facilitates changing axle hardware (nuts and washers) since they don't need to pass over connectors, and it makes the cable less likely to suffer damage as they exit from inside the bicycle dropout.
  3. Round axle: Most ebike hub motors have larger 12mm/14m axles that are ground flat on the sides, but this results in the axle of the motor sitting further down in the dropout than a normal bicycle hub causing the disk rotor to not fit all the way in the caliper, and it often requires filing the dropouts to get the motor in. The G311 and G310 hubs have a 10mm round axle to avoid that, with integral anti-rotation washers to transmit spinout torque further down the dropout slot.
  4. Easy Disassembly: Many models of geared motor are assembled with two halves that thread together or joined with an interference fit, and are nearly impossible to open up without special tools. Bafang has done an excellent job in making motors that are bolted together and can easily come apart for service and repair.
  5. Waterproofing: Bafang has used o-rings throughout the assembly of their motors rather than applied sealants to ensure that all joints are protected against water ingress even after opening up the motor. They also have shaft seals over all bearings, and as a result water ingress into these Bafang hubs is a rare occurance indeed.
  6. Internal Speedometer Sensor: The motors have a 6 pole speedometer built into the motor shell, allowing the use of a Direct Plug Cycle Analyst for speed readings rather than requiring a -DPS device with separate spoke magnet and speed sensor. 
  7. Cassette Freehub compatibility: The G310 motor uses a shimano compatible cassette freehub body instead of a threaded freewheel, making it compatible with modern bicycle drivechains, including the 9 and 10 speed systems that are fast becoming the norm.

Power Levels, Speed, and Winding Options

The nominal power rating of 250W from Bafang doesn't mean a whole lot. These motors can put out 30 Nm of torque continuously and upwards of 40-45 Nm for short times without overheating. At higher speeds (like 40 kph) this translates into a solid 600-700 watt continuous power levels, while the corresponding power at lower speeds is more like 200-300 watts. Usage beyond 50-60 Nm is not recommended as there is real risk of stripping the gears.

We stock the motors in 3 different winding options, a very fast (7T) ideal for 16" rims, a fast wind (8T) intended for 20" wheels, and a standard wind (10T) that is appropriate for 26"/700c builds. With this combination of winding choice and wheel diameter, you will achieve cruising speeds of 30-35 kph with 36V battery packs and 40-45 kph with 48V/52V packs.

For exact performance details with any combination of wheel size, battery voltage, and motor winding please have a look at our motor simulator page (Standard Wind, Fast wind, VeryFast wind). We have these hubs thermally modeled as well so you can see if your application needs will be pushing them to overheating. In cases where the motor will be prone to overheat with your setup, then we would recommend a larger and more poweful geared motor like the eZee hub. 

Internal Construction

The G310/311 motors have an unusual contruction. Most geared hub motors have an outrunner motor spinning the sun gear of a single stage planetary gear reduction to the shell. The G31X motors instead have a small inrunner motor turning a helical cut sun gear. It does not have a common axle that runs through the entire motor. This allows for a smaller diameter sun gear than a typical configuration. That small gear diameter combined with the double stage on the planet gears produces an impressive 11:1 total reduction ratio allowing the smaller / lighter motor to still produce a decent torque at the wheel.

The helical first stage gears allow for a smooth transmission with almost no vibration or noise. But this spiral cut also produces a small axial force component that must be countered with thrust bearings. These needle thrust bearings are visible under the planet gears, and care should be taken not to loose them during any motor disassembly.

Wiring Pinout

These motors are terminated with a waterproof HiGo Z910 overmolded connector. These have the the phase and signal wires in a single connector which is great for convenience. However, it does not allow for the user to play around with hall and phase pinouts which is often required when pairing with a 3rd party motor controller. Fortunately most controller and motor vendors using this connector have chosen a compatible pin configuration.  The 6th signal wire is used for the speedometer, which pulses 6 times per wheel revolution. On the matching Grinfineon controllers, this speed signal is routed back to the CA-DP plug for the Cycle Analyst to have a speed readout. 

Washer Stack and Torque Arm

These motors do not have intrinsic flats on the axle for preventing axle spinout. They have a keyed washer that fits inside the forks and has a tab that engages with the bottom of the dropout slot. This gives the same kind of leverage against spinout as you'd have from a large 14mm flatenned axle, with the benefit that the motor axle center is in the same location as a normal bike hub, and without the oversized M14 nuts and washers that don't fit inside the lawyer lips.On the downside there is no facility for external torque arms to move the reaction torque further up the fork blade, all the torque is resisisted at the dropout slot via these washers.

Most bicycle dropouts have more than adequate strength to resist spinout from the washer tabs. However, if you have a front fork with aluminum dropouts, there is not much margin between the motor torque and dropout failure. For installation on bikes with alloy dropouts, we have a specially made inside torque arm that replaces the left side anti-rotation washer and has an arm that bolts to a fender eyelet. In addition to a regular washer, lock washer, and barrel nut, there is also a thin 1mm spacer washer that is available in the washer stack. This can be placed under the tabbed anti-rotation washer to give a bit more clearance between the side of the motor and the fork, which can sometimes be handy for disk rotor alignment, fitting 9 or 10 speed freehubs, or giving a bit more room for the wide part of the fork blade to not rub against the side of the motor.

Disk Brake Compatibility

Both the G310 and G01 motors have the mounting holes for a 6-bolt ISO disk rotor. If you aren't using a disk, then be sure to leave the plastic spacer and M5 bolts in place to prevent water ingress into the disk mounting holes. Ideally the disk rotor mount will be 10mm from the axle end on a front motor and 15mm on the rear. The G01/G310 motors are slighly less than this. Most disk calipers have sufficient left/right play in their mounting to accomdoate that, but if not then the 1mm spacer washer can be placed on the axle to give a bit more room.

Controller Options and eRPM

The G310 motor have a large 11:1 gear reduction and 16 magnet rotor producing 88 effective pole pairs, which is higher than the 40-50 effective poles in most other geared motors. In many situations this puts the electrical commutation frequency (eRPM) at cruising speeds that is outside the range of sensorless controllers. The table below shows the eRPM of the G310 motors at different speed and wheel size combinations so you can see if your particular controller will have an issue or not in sensorless mode. The motors have the Higo Z910 conncector on them and we supply a plug-in compatible 20A Grinfineon controller with the kits which is connected in sensored mode for no eRPM limit.

Bicycle Fitment

The combination of cassette freehub, 138mm and 100mm axle lengths, 10mm round axles, and ISO disk rotor compatilbility makes these motors a great fit for modern bike conversions. However there are a few things to watch out for.

  • Dropout Filing: Even through these motor axles are smaller than normal hub motors, it's still the case that many dropout slots will still be slightly too narrow for the axle to slide in. If that's the case, you will need to file the dropout slot wider until the 10mm round axle can drop into place. 
  • Axle OLD: The rear G310 motor has a 138mm rather than 135mm axle OLD. Spreading a 135mm standard bike dropout to 138mm is generaly no problem, that's just just 1.5mm on either side and can stretch by hand with minimal force. 
  • Cassette Stack Height: With a 9 or 10 speed freehub, an additional 12mm washer is sometimes needed on the right side to accommodate the larger gear stack and have the chain clear the dropout. This will increase the OLD to 139mm. 
  • Fork Rubbing: The G311 front motor is narrower than most of the other front geared motor models and is less likely to interfere with wide fork blades than say the G01 model. Below is a 1:1 scale side view drawing that can be printed and cut out to check the fit if you are concerned about fork clearance. If the interference is on the order of 1mm or so then the addition of an inside washer will space the fork out sufficiently far to clear.

CAD Drawings

Videos

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